Semipermeable membranes have long been used for the purification of water. Such membranes are useful, for example, in the removal of dissolved inorganic salts. In such processes, water containing the dissolved salts is held under pressure against a membrane which passes water but does not pass the salt ions.
Polyamide reverse osmosis membranes have been used in the form of hollow filaments or flat films in cartridges for the purification of saline and brackish waters. It is often necessary, however, to store the reverse osmosis membranes for extended periods before they are put into use, either due to extended periods for transportation or inventory requirements. The membranes are normally stored in a wet condition, and the water used for storage frequently contains additives which decrease the pH of the aqueous solution. For example, bacteriostats such as formaldehyde are typically added in concentrations of about from 5 parts per million to 1 percent by weight to inhibit the growth of bacteria during storage. The formaldehyde, however, degrades to formic acid, which results in the decrease in pH of the storage solution. With increasing degrees of acidity, the polyamide membranes tend to degrade, thus reducing their performance capabilities and effective life.